As a cooling device which cools an electronic device or other heat generating member, a heat pipe which circulates a working fluid which is sealed inside it through a loop shaped pipe and utilizes a phase change of the working fluid to transport heat is known. In general, a heat pipe is a cooling device using two-layer flow of a gas and liquid which circulates a liquid phase cooling solution by using a liquid transport pump and which makes the cooling solution boil by a cooling device or heat receiver and uses the latent heat of evaporation to realize a high cooling performance. A cooling device which uses a liquid transport pump is suitable when the distance between the heat receiving part and the heat dissipating part is long and the heat transport distance is large or when the heat receiving part is made thinner and the flow path is made narrow like with a microchannel or otherwise when the pressure loss of the circulating route is large.
On the other hand, a cooling device using a loop type heat pipe is known which does not use a liquid transport pump, but uses the capillary force of a porous member (wick) provided at an evaporator to circulate a working fluid. A loop type heat pine uses the capillary force of a porous member in an evaporator to circulate a working fluid, so motive power for a heat transport pump etc. is not required and the vapor pressure inside the evaporator can be used to transport heat to a condenser at a distant location. Such a loop type heat pipe is, for example, disclosed in Japanese Laid-Open Patent Publication No. 2009-115396A and Japanese Laid-Open Patent Publication No. 2007-247931A.
The loop type heat pipe which is disclosed in Japanese Laid-Open Patent Publication No. 2009-115396A is characterized by an evaporator structure which has a plurality of wicks inserted in the horizontal direction and which is thinner and can be increased in evaporation area (surface area of wicks) compared with the case of a single wick. Further, the loop type heat pipe which is disclosed in Japanese Laid-Open Patent Publication No. 2007-247931A is characterized by a structure which has a wick superposed on a heating surface and which is enlarged in evaporation area and improved in performance by provision of relief shapes facing the heating surface and wick.
However, in the loop type heat pipe which is disclosed in Japanese Laid-Open Patent Publication No. 2009-115396A, the evaporator is made thinner, so it is difficult to make the liquid phase working fluid uniformly permeate the wide area porous member and evaporate, part of the porous member dries out resulting in circulation of the working fluid becoming unstable, and the performance fails. Further, in the loop type heat pipe which is disclosed in Japanese Laid-Open Patent Publication No. 2007-247931A, realizing greater thinness is easy, but when the heat generating member increases in the amount of heat generated and the amount of evaporation increases, it becomes harder to supply liquid to the tip of the wick, dry out occurs, the evaporation area is reduced, and the cooling performance remarkably falls.